1. Field of the Invention
The invention relates to a device for optical heterodyne or homodyne detection of an optical signal beam. The device includes a local oscillator, a beam-combining element for combining radiation from the local oscillator with radiation from the signal beam, at least one opto-electrical converter for converting combined optical radiation into an electric signal, a plurality of adjustable elements influencing the state of polarisation and having a limited control range and a control circuit for adjusting the elements influencing the state of polarisation. The control circuit is adapted for endless control of a state of polarisation. The invention also relates to a receiver for receiving optical signals, provided with such a device.
2. Prior Art
As compared with direct detection of an optical signal beam, heterodyne or homodyne detection provides considerable advantages relating to the signal-to-noise ratio and discriminating from background radiation. The principle of heterodyne detection of optical radiation has been extensively described in the Article "Optical Heterodyne Detection" by O. E. DeLange in the Journal "IEEE Spectrum" of October 1968, pages 77-85. As has been stated in this Article, it is important that the states of polarisation of the modulated signal beam and of the local oscillator beam correspond as much as possible.
Optical transmission systems make use of optical waveguides or optical fibres. These waveguides or fibres may be dozens to several hundred kilometers long and are subjected to uncontrollable external influences such as temperature, stress and pressure variations. The state of polarisation of the radiation propagating through the fibre is disturbed by these influences. A signal beam which is linearly polarized at the input of the fibre will generally have an elliptical state of polarisation at the output. Since the external influences vary with time, the ellipticity and the orientation of the polarisation ellipse also vary.
In order to compensate for the signal beam state of polarisation varying with time, a polarisation control of the signal beam or of the local oscillator beam is necessary. The Article "Endless polarisation control experiment with three elements of limited birefringence range" by R. Noe in the Journal "Electronics Letters", Vol. 22, No. 25 (1986), pages 1341-1343 describes such a polarisation control in which the state of polarisation of the signal beam can be endlessly compensated by means of three adjustable elements influencing the state of polarisation and each having a limited control range. Within the scope of the present invention, endless control is to be understood to mean a compensation control enabling the state of polarisation of the signal beam to be tracked in the case of a continued change of this state in the same direction so that the difference between the states of polarisation of the two beams does not become so large at any instant that there is serious signal loss of the signal ultimately detected by the detection device. For a polarisation control in which all adjustable elements have a limited control range this means that, if one of the elements influencing the state of polarisation tends to exceed the limit of its control range, it must be possible to readjust this element, whilst the polarisation compensation and hence the signal reception is largely maintained by suitable manipulation of the adjustments of the other elements.
It is to be expected that optical transmission systems will be widely used, not only for transmitting information between central stations but also for distributing information from a central station to, for example subscriber terminal stations. It is particularly important for such a terminal station that the detection device present therein is relatively inexpensive and hence is equipped with a minimum number of controllable components and the associated control circuits.